DESYNAPSIS AND CHROMOSOME BREAKS IN A MUTANT OF POSUM SATIVUM

An X-ray induced mutant of Pisum sativum shows considerable meiotic irregularities caused by a reduction of chiasma frequency and the occurrence of chromosome breakage during the first meiotic prophase. In the subsequent stages univalents, fragments and bridges arise, resulting in abnormal chromosome distribution and complete sterility of the mutant. The simultaneous occurence of reduced chiasma frequency and chromosome breakage seems suitable to test the hypothesis of LEWIS AND JOHN5 that chromosome breakage might be produced by errors in the cross-over process. The following results of our investigations are regarded as a confirmation of this assumption. 1. 1. Both chromosome breakage and reduced chiasma frequency take place in meiotic prophase during the four-strand stage. In both cases 2 of the 4 chromatids are involved in the exchange process, and in both cases the exchange is complete and reciprocal. 2. 2. The investigation of anthers being fixed at two different developmental stages of ontogenesis-immediately after entering flowering period and 4 weeks later-reveals remarkable differences as far as the amount of reduced chiasma frequency and aberration frequency is concerned. Both these factors change in the same way: the greater the number of chiasmata, the higher the number of aberrations, and vice versa. 3. 3. A comparison between fragment size at metaphase I and chiasma distribution in the bivalents shows that chromosome breakage and chiasma formation are located in the same chromosome regions. Because of the predominance of sister-chromatid reunions, the possibility is discussed that ineffective chromosome pairing might be responsible for both the reduced chiasma frequency and the occurence of fragments and bridges. © 1969.